The use of metakaolin in cement is known. For example, U.S. Pat. No. 4,793,861 describes a cement-based product which is reinforced with glass fibers having good resistance to alkaline environments. The product contains, for each 100 parts by weight of cement, about 10 to 40 parts by weight of metakaolin, the latter exhibiting a reactivity to the modified Chapelle test greater than 500 mg of CaO per gram of metakaolin.
U.S. Pat. No. 4,842,649 describes a blended hydraulic cement composition composed of portland cement, slag, pozzolans including metakaolin, and admixtures including potassium carbonate and water reducing compositions.
U.S. Pat. No. 4,975,396 describes a process for producing reinforced cementitious compositions in which the following constituents are mixed in the aqueous phase in the following order: about 35-55 parts by weight of water mixed with about 3-12 parts of a polymer, by weight of dry polymer; up to about 5 parts of a water-reducing auxiliary agent and/or a liquefying agent; from about 15-30 parts of metakaolin; from about 50-120 parts of silica sand; and about 100 parts of cement. Continuous mixing is maintained until a homogeneous, thixotropic paste is obtained. Then between 2 and 15% by weight of alkaline-resistant glass fibers, relative to the weight of the paste, is introduced into the paste.
U.S. Pat. No. 4,994,114 describes method for selecting a pozzolan (for example metakaolin) for incorporation into a composite material comprising cement and glass.
U.S. Pat. No. 5,167,710 describes a process for making a cement mixture containing fibers wherein a paste is formed by mixing cement and, per 100 parts by weight of cement, approximately 5 to 20 parts by weight of a first pulverized material of which the grains have an average diameter of between 1/5 and 1/10 of the average diameter of the grains of the cement and approximately 20 to 35 parts by weight of water. The paste is then mixed with reinforcing fibers. The paste may also include a second pulverized material the average grain diameter of which is between 1/5 and 1/10 of the average diameter of the first pulverized material.
U.S. Pat. No. 5,372,640 describes cement-based products reinforced with alkali-resistant glass fibers that become almost insensitive to aging when 30 to 40 parts by weight of a metakaolin composition are added for each 100 parts of cement.
U.S. Pat. NO. 5,624,489 describes a conversion-preventing additive for high-alumina cement-based compositions, the additive comprising: siliceous pozzolanic powder, e.g. zeolite, granulated blast-furnace slag, fly ash, silica fume, rice hulls, metakaolin; inorganic salts containing sodium or potassium cations and sulphate, carbonate, nitrate, silicate, phosphate, chloride or bromide anions, and optionally other chemical admixtures, e.g. superplasticizers.
U.S. Pat. No. 5,626,665 describes cementitious systems comprised of gypsum, calcined clay, and clinker.
Pozzolans are finely divided materials which can react with alkali to form cementitious products. The fine particle size and large pore volume of pozzolans, however, can lead to an increase in water demand. In cement-based systems, the addition of extra water can reduce the performance of the system by reducing the strength and increasing the permeability of the resultant cement-based structures. The diminished strength is undesirable for several reasons. Initially, delay in early strength development results in surface cracking due to evaporation. Secondly, jobs take longer because the concrete form must remain in place substantially longer, and finishing is delayed.
Reactive pozzolans can be made by dry grinding metakaolin using ball milling. However, ball milling is an expensive and time consuming process. In this connection, ball milling often requires 6 hours of milling time. Such long processing times restricts the commercial manufacturability of dry milled pozzolans. Dry milling is not therefore frequently employed on the raw metakaolin. However, the addition of metakaolin to finishing mills or mills for clinker or slag grinding (dry milling) is employed for incorporating pozzolans into cement-based systems.
Nevertheless, there is still a need for pozzolans having improved activity to provide cement-based systems and alkali activated systems having lower water demand and higher compressive strength, as well as improved flowability as a dry powder and a higher bulk density to reduce shipping and storage costs.